This chapter evaluates sexual offender recidivism. Recidivism in sexual offenders is critical for two reasons. The first reason is that based on studies of recidivism, risk assessment instruments are used to predict future recidivism. Second, recidivism is used as a measure of treatment outcome and rehabilitation of the sexual offender. The understanding of sexual offense recidivism is vital for anyone who is engaged in the assessment and treatment of sexual offenders. In more recent years, the focus on sexual offense recidivism appropriately has been directed by the impact on individuals of their victimization by sexual offenders. All of this supports efforts on treatment and rehabilitation to reduce recidivism. When considering sexual offense recidivism and treatment outcome studies from an empirical standpoint, there are some critical considerations to evaluate. There is a consensus that sexual offenses are underreported, and this may affect the statistical estimates in recidivism and treatment outcome studies. To compensate for underreporting, the definition of an act of recidivism includes rearrest as well as reconviction.